Th2

T helper 2 (Th2) cells are a subset of CD4+ T helper cells that produce interleukin-4 (IL-4), interleukin-5 (IL-5), interleukin-13 (IL-13), and interleukin-10 (IL-10). Th2 differentiation is driven by antigen presentation via MHC II combined with IL-4 signaling (often from activated mast cells, basophils, or prior Th2 cells in an autocrine loop). The transcription factor STAT6 phosphorylated by IL-4 is critical for Th2 development. Th2 cells promote: (1) B cell differentiation and class-switching to IgE and IgG4 (humoral/antibody responses); (2) eosinophil recruitment and activation via IL-5; (3) mast cell activation; (4) alternative macrophage activation (M2 phenotype); (5) suppression of Th1 responses (IL-10 and IL-13 inhibit IL-12 and IFN-gamma). Th2-mediated immunity is important for parasitic infections (helminths) but also drives allergic and atopic responses (asthma, allergic rhinitis, eczema, anaphylaxis). Th1 and Th2 are mutually antagonistic (Th1 produces IFN-gamma which inhibits Th2 IL-4 production; Th2 produces IL-10 which inhibits Th1). This Th1-Th2 balance is critical for appropriate immune response; skewing toward Th2 predisposes to allergy/parasitic disease; skewing toward Th1 predisposes to autoimmunity and cellular immunity. On boards, link Th2 to allergies, IgE, eosinophilia, and parasitic infections.